Noise-canceling microphone



Sep. 16, 1952 R, K, DUNCAN 2,611,035

NOISE-CANCELING MICROPHONE Filed Jan. 51, 1950 :Snventor 5 (Ittorneg Patented Sept. 16,1 1952 Noise-CANGELING MiCRoPI-IONE Robert K. Duncan, Woodburn, Ind., assgnor to `Radio Corporation of America, a corporation of Delaware Application January 31, 1950, Serial No. 141,483

Claims. (cl. 179-,1

This invention relates' to electro-acoustical a paratus, 'and more particularly to an improved, noise-cancel'ling, close-talking microphone.

The close-talking microphone referred to herein is one which is useful in l-ocations whererthere is considerable noise, such as in many military Operations. Such -a microphone is so 'constructed mechanioally and acoustically that it provides -a high speech-to-noise ratio, depending for its operation on -an extremely small talking distance as between the lips of the user and the -microphone diagram or sound 1receptive orifice.

My Vpresent invention relates to the second order pressure gra-'Client `responsive close-talking 'microphone the conventional structure of which includes two first order, pressure -gradient responsive microphone units arr-anged one above the other -and 'having their diaphragms located as close together as would lhe 'possible without the one interfering With the 'other in respect to sound Waves impinging directly on both of them. Electric-ally the units are connected so that their output voltages oppose each other 'and when equal sounds are picked up by each unit their outputs will cancel. In use, a person talks more 'directly into one of the units thanthe otherl unit so that the miorophone will .pradu-ce 'an output from the useful sound wave energy. Structures of this type are found' to -possess certain disadvantages in the respect that the unit to which the useful sound wave energyis not directed acts as an electrical load on the 'other unit With the -result that the microphone suff-ers a loss in useful output. In addition, the cancellation of extraneous high frequency sounds is not as effective due to the vli-mitation of spacing of the two diaphragms. .A further disad- Vantage results from 'the f'act that with the conventional microphone arrangement, it is vpossible to vposition the sound source less than one half .inch from the directlydriven {diaphragm resulting in poor response' from exceedingly pronounced blast effect from such sounds as Bj' "P, etc., ;and from accumulation of moisture on the diaphragm from the breath of the 'person using the microphone. 5

The primary object of my present invention is to provide -an improved noise-cancelling, closetalk'ing microphone ,which will overcome the above 'as well as other disadvantages'of prior .art close-talking mcrophones.

It is also an object of my present invention to' provide a close-talking microphone which will be more effective in the .cancellation o f .extraneous sound in the high frequency range.

Another object of my present invention is to provide-a close-talking microphone which will provide imp-roved response to useful sound Waves. effect :canceillation of 'extraneous sound waves over a wider range of frequences ;and which 'will have a larger output.

A further object. of my present invention is to provide an improved .close-talking microphone which is simple 'and easy to construct .as Well as 'highly -efiicient in operation.

'In accordance with my present invention, I provide a noise-cancelling, close-talking microphone ,comprising :two first order, pressure gradient 'responsive transducive units disposed facing .each other in spa-ced apart relation along a common axis so that `the planes of their vibratile .elements are substantially parallel. The vibratile elements are mounted for movement in opposite fdirections in response to sound Waves impinging thereon from a closely disposed sound source directed between them. 'The units have their signal outputs `-electrcal1y connected in series so 'that the voltagesdeveloped respectively by said units will be additive when the vibratile elements move in opposite-zdrections |and will oppose each other Vwhen the vbratile elements move in the same direction.

The novel featurescharacteristic of my present invention, as well as additional objects and adv-antages thereof, ,will he'better vun'clerstood from the vfollowing detailed description when read in connection with the accompanying drawing inwhich:

Figure 1 is a front elevation .of a noise-can- ,ce1ling, |close-talking microphone -in accordance `with my present invention,

Figure 2 is a side elevation 'of the mi'crophone shown in Figure 1,

Figura 3 is a front view of one of the transducer units shown in Figura 1,

Figura 4 is a sectonal view of the microphone transduoer units, 'taken along the line 4-4 of Figure 1, and

Figure 5 is |a diagram of the .electrical circuit connecting the outputs of the two transducer units. 1

Referring more' particularly to the drawings, wherein similar reference characters designate corresponding :parts throughout, there is shown a noise-cancellng, close-.talking microphone I, in aocordance with a preferred embodiment of mypresent nvention, comprising, generally, a pair of conventi-onal first order, pressure gradient responsive transducer units .ii and a hand support member 5.

'I'he transducer units 3 are mounted on the support member 5 along a common axis with their front faces or ends 1, that is, the ends into which sound Waves 'are normally directed, .disposed in closely spaced apart relation. Each of the transducer units 3 is of the electrodynamic type and comprises a vibratile element or diaphragm Il mounted for movement in response to the pressure gradient component of sound Waves impinging on opposite sides thereof. At-

tached to the diaphragm ll is a coil 13 which is disposed in a magnetic field provided by a suitable magnetic structure and operative in a manner well known in the art.

The magnetic structure of each of the trans- 'ducer units 3 illustrated and described herein comprises a frustro-conical permanent magnet |5 having the base thereof mounted ona base plate l'l within a recess |9 provided therein. A central or inner circul'ar pole member 2| is mounted on the apex end of the magnet 15. An outer pole plate member 23 has a circular aperture 25 within which the inner pole member 2| is concentrically disposed in spaced relation to the outer pole plate 23 to provide'an annular air gap 21. The outer pole plate 23 is supported by a plurality of rod-like members s'which support it from and connect it with the baseplate ll. The rod-like members 29 may be connected` to the outer pole plate 23 and .the base plate H in any suitable manner which will provide a good magnetic fiux conducting path between the plates.

The diaphragm ll' is supported at its periphery by the outer pole plate 23 in a manner to permit it to freely vibrate in response to sound waves impinging thereon and to dispose the coil I 3 within the air gap 21 for freedom of movement therein. Thus, the diaphragm H together with the coil |3 comprise a movable conductor disposed within a magnetic field provided by the L magnetic structure. A protecting cover 3| of non-magnetic material, which is provided with a central opening 33, ismoun'ted in front of the diaphragm II in spaced relation thereto. Another protecting coverf is mounted in back of the air gap 21 and is provided with a plurality of apertures 31 disposed opposite to the air gap 21. screens 39, 4| fare disposed, respectively, within the central' opening 33 and the apertures 31 to prevent dustand magnetic dirt particles from interfering with the movement of the diaphragm ll and the coil l3. Suitable terminals 43 are provided on the front protecting cover 3| to which one end of leads 45 from the coil I3 are connected, and to the other end of which wires 41 are connected for connection with an external electrical circuit.

The distance of separation between the transducer units 3 should be rather small to be effective, preferably of the order not to exceed 1A; wavelength at the highest frequency in the range to which the units are to be responsive;

The units 3 have their respective outputs connected in series so that the voltages developed are additive when the diaphragms ll move in opposite directions, and oppose each other when the diaphragms move in the same direction. Thus, it will be recognized by those persons skilled in the art, that since both units are first order, pressure gradient responsive units, their diaphragms H will be moved in opposite directions only when they respond to sound waves originating from a source closely disposed to the units and directed between them. If the sound source is an appreciable distance away from the units in a plane passing between them, there will be no output from 'the units for, in that case, the sound source will be disposed substantially in the null plane of each of the diaphragms. If, however, the sound source is disposed on either side of the plane passing between the units 3, their diaphragms l I will be caused to move in the same directions, in which case, their outpu'ts will oppose each other and effectively cancel. It Will also be recogm'zed that When the diaphragms ll are driven in opposite directions, the combined outputs of the units is almost twice that obtained from the conventional second order, pressure gradient responsive microphone wherein 'the one unit acts as an electrical load on the other directly driven unit.

Thus, the arrangement provided by my present invention will be found to overcome the disadvantage resulting from placing the sound source too close to the diaphragms, as in the case with the conventional type. Because of the physical size and arrangement of the units, it is practically impossible to dispose the sound source closer than approxim'ately 1A; inch and thereby reduce the blast effect mentioned above. In addition thereto, since 'the voice of the user is directed between the units, there will be less tendency for moisture to accumulate on the diaphragms which otherwise would interfere with efiiciency of operation.

While I have shown and described but a single embodiment of my present invention, it will also be recognized that various changes and modifications are possible within the spirit of my invention. Therefore, I desire that the particular form of my present invention described herein shall be considered as illustrative and not as limiting.

What is cl'aimed is:

1. A sound translating device comprising a, pair of dynamic microphone units, each of said units having a vibratile member mounted for movement in response to the first order gradient of the sound pressure, said vibratile members being mounted in spaced apart relation facing each other for movement in opposite directions in response to sound Waves impinging thereon from a closely disposed sound source and directed betwcen said members, and means for eifectively connecting the signal outputs of said units whereby the voltages developed respectively by said units will be additive when said vibratile members move in opposite directions and will oppose each other when said vibratile members move in the same direction.

2 A sound translating device according to claim 1, wherein said vibratile members are spaced apart a distance not to exceed 1A wavelength at the highest frequency in the range to which the units are to be responsive.

3: In sound translating apparatus, the combination of means providing separate magnetic fields, a pair of movable conductors each responsive to the first order gradient of the sound pressure and vibratably mounted in separate ones of said fields, said conductors being disposed in closely spaced relation to and facing each other for movement in opposite directions in their respective fields in response to sound waves implngmg thereon from a closely disposed sound source and directed between said conductors. and means connecting the signal outputs of said conductors to additively combine the voltages developed respectively therein when said conductors move in opposite directions in said magnetic fields and to combine said voltages in opposing relation when said conductors move in the same direction in said magnetic fields.

4. A sound translating device comprising a Vpair of microphone units each of which comprises (1) a magnetic structure having an air gap with a magnetic field 'therein, (2) a conductor mounted in said field for vibration in response to the first order gradient of the sound pressure, said units being mounted in spaced apart relation along a common axis with their respective conductors fiacing each other in closely spaced apart relation, and means for so connecting the signal outputs of said units that their respective voltages will be additive when said conductors move in opposite directions in their respective magnetic fields and will voppose one another when said conductors move in the same direction in said magnetic fields.

5. A sound translating device comprising-a pair of dynamic microphone units each having a vibratile member mounted for movement in response to the first order gradient of the sound pressure, said units being mounted in spaced apart relation along a common axis and having 6 'their respective vibratile members disposed facing each other with their axes of maximurn response substantially colinear, and means for so connecting the signal outputs of said units that their respective voltages will be additive when said vibratile members move in opposite directions and will oppose each other'when said vibratile members move in the same direction.

ROBERT K. DUNCAN.

REFERENCES cI'rED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,54'7,"740 Fressenden July 28, 1925 2,043,416 VLuez June 9, 1936 2,198,424 Baumzweiger Apr. 23, 1940 2,301,744 Olson Nov. 10, 1942 2,305,599 Bauer Dec. 22, 1942 2,350,010 Beckley May 20, 1944 2,361,656 Rogers Oct. 31, 1944 2,3%,222 Foldy Mar. 5, 1946 2,544,536 Kettles Mar. 5, 1951 

